Add grep functionality

This PR adds `grep` and `listTests` commands to Intern. `grep` allows tests to be filtered by test ID. `listTests` lists all the tests that Intern will run, post filtering if `grep` is used. `grep` works from the command line and may be set in the test config file. `listTests` only works from the command line. See #123.
Seems like this check should be done earlier (later in the function) to avoid running beforeEach/afterEach for tests that have been skipped by grep. However, this will lead to an inconsistency in execution if someone programmatically chooses to mark a test as skipped from within the test body. Thoughts?